In these years, mobile communication terminals including cellular phones marked a rapid market increase. For purposes of portability, these terminals are desired to be of small size and light weight. For the manufacture of small-sized, lightweight terminals, the size and weight of electronic parts used herein must be reduced. Surface-acoustic-wave (SAW) devices, specifically SAW filters are often used in high and intermediate-frequency portions of such terminals because SAW devices are advantageous for size and weight reductions. The SAW devices include interdigital electrodes formed on a surface of a piezoelectric substrate for exciting, receiving, reflecting and propagating surface acoustic waves.
Important characteristics of the piezoelectric substrate used in SAW devices include a propagation velocity of surface acoustic waves (SAW velocity) and an electromechanical coupling constant (k.sup.2). The characteristics of some typical known substrates for SAW devices are given in Table 1. For the detail of these characteristics, reference should be made to Yasutaka SHIMIZU,"Propagation characteristics of SAW materials and their current application," Journal of the Electronic Information and Communication Society A, vol. J76-A, No 2, pp. 129-137 (1993). Hereinafter, the piezoelectric substrates for SAW devices are referred to using the designations in Table 1.
TABLE 1 Com- Propagation SAW velocity k.sup.2 Designation position Cut angle direction (m/s) (%) 128LN LiNbO.sub.3 128.degree. X 3880-3920 5.6 rotated Y 64LN LiNbO.sub.3 64.degree. X 4330-4360 11 rotated Y LT112 LiTaO.sub.3 X 112.degree. 3220-3260 0.72 rotated Y 36LT LiTaO.sub.3 36.degree. X 4100-4160 5.0 rotated Y ST quartz quartz ST X 3130-3155 0.17
As seen from Table 1, 64LN and 36LT have a SAW velocity of higher than 4,000 m/s and are suitable for constructing filters in high-frequency portions of terminals for the following reason. For mobile communications as typified by cellular phones, a variety of systems have been used in the world. All these systems utilize frequencies of about 1 GHz. Then the filter used in the terminal high-frequency portion has a center frequency of about 1 GHz. In the case of SAW filters, the center frequency is approximately in proportion to the SAW velocity of the piezoelectric substrate used and in inverse proportion to the width of fingers of electrodes on the substrate. For higher frequencies, those substrates having a high SAW speed, for example, 64LN and 36LT are preferred. Also the filter in the high-frequency portion is required to have a wide band such as a pass-band width of more than 20 MHz. To broaden the band, the piezoelectric substrate must have a greater electromechanical coupling constant (k.sup.2). From this aspect too, 64LN and 36LT are often used.
As the intermediate frequency of mobile communication terminals, a frequency band of 70 to 300 MHz is generally used. When the filter having a center frequency in this frequency band is constructed by a SAW filter utilizing 64LN or 36LT as the piezoelectric substrate, the width of fingers of electrodes on the substrate must be substantially greater than that in the filter used in the high-frequency portion.
This is illustrated by referring to the estimation of specific values. A SAW transducer constructing a SAW filter has an electrode finger width d, the SAW filter has a center frequency f.sub.0, and the piezoelectric substrate used has a SAW velocity V, whose relationship is generally expressed by the equation: EQU f.sub.0 =V/(4d) (1).
When a SAW filter having a center frequency of 1 GHz is constructed using a piezoelectric substrate having a SAW velocity of 4000 m/s, the electrode finger width is calculated from equation (1) to be: EQU d=4000(m/s)/(4.times.1000(MHz))=1 .mu.m.
On the other hand, when an intermediate frequency filter having a center frequency of 100 MHz is constructed using a piezoelectric substrate having a SAW velocity of 4000 m/s, the electrode finger width is calculated to be: EQU d=4000(m/s)/(4.times.100(MHz))=10 .mu.m.
This indicates that the necessary electrode finger width is 10 times greater than in the filter of the high-frequency portion. A greater electrode finger width means that the SAW device itself becomes larger. In order that the SAW intermediate frequency filter be small in size, a piezoelectric substrate having a low SAW velocity V must be used as understood from equation (1).
As shown in Table 1, ST quartz is known as the piezoelectric substrate having a relatively low SAW velocity. ST quartz is known to have a SAW velocity of 3130 to 3155 m/s, although this value cannot be regarded definite because the effective SAW velocity of a piezoelectric substrate is affected by the structure of electrodes formed thereon. This SAW velocity is about 3/4 of that of 64LN and 36LT and is advantageous for purposes of size reduction.
For the above-described reason, most SAW filters for intermediate frequency portions in mobile communication terminals are constructed of ST quartz piezoelectric substrates. As seen from Table 1, the ST quartz substrate has an electromechanical coupling constant (k.sup.2) of 0.17%, which is extremely low among other piezoelectric substrates. A small value of k.sup.2 means that only a filter having a very narrow pass-band is constructed.
Analog systems are mainly employed in the conventional mobile communications, namely cellular phones. Since these systems have a very narrow channel width, for example, of 12.5 kHz in the NTT system in Japan, 30 kHz in the AMPS system in the U.S., and 25 kHz in the TACS system in Europe, the low electromechanical coupling constant (k.sup.2) of ST quartz substrates gives rise to no problem. However, from the standpoints of effective utilization of frequency resource and compatibility with digital data communication, a digital mobile communication system was recently developed. The digital system has matured to a practical level and become widespread. The channel width of the digital system is very broad, for example, 200 kHz in the cellular phone GSM system in Europe and 1.7 MHz in the cordless phone DECT system. When an intermediate-frequency filter with such a broad band is constructed by a SAW filter, it Is difficult to manufacture the filter using ST quartz substrates.
As described above, the prior art SAW devices have the problems that the use of a piezoelectric substrate having a high electromechanical coupling constant allows for a wider pass-band, but leads to a greater device size because of the greater SAW velocity of the substrate; and that when a substrate having a relative low SAW velocity is used for the purpose of reducing the device size, the pass-band width cannot be broaden because of a too low electromechanical coupling constant. Either case fails to achieve satisfactory characteristics as the intermediate-frequency SAW filter.